Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 4 Lipids and Ketones
FIGURE 4-5 Densitometric scan of an electrophoretogram of canine
plasma lipoproteins. The scan is typical of a fasted dog. In a fed dog, an
additional peak as a result of chylomicrons would be present at the origin.
Abbreviations: HDL, LDL, and VLDL, high-, low-, very low density
lipoproteins, respectively.
cholesterol ( Stein and Meyers, 1994 ). Such empirical
methods may be species specific. For example, the preceding
method, though valid for human plasma, does not work
for dog plasma ( Rhodes et al ., 1992 ).
B. Apolipoproteins
The protein components of lipoproteins are called apolipoproteins.
Some apolipoproteins are found in only one class
of lipoproteins, whereas others can be found in multiple
classes. Although there are species variations in the amino
acid sequences of apolipoproteins, individual apolipoproteins
in the domestic species are quite similar. The main
classes of apolipoproteins are designated with a letter (A
through E), sometimes followed by a number to indicate a
distinct subclass. The main classes and subclasses of apolipoproteins
found in domestic animals are A-I, A-II, A-IV,
B 48 , B 100 , C-I, C-II, C-III, C-IV, and E. Characteristics of
these apolipoproteins are listed in Table 4-2 .
The B 100 apolipoprotein, synthesized in the liver and
part of VLDL, is one the largest polypeptide chains in
mammals, having a molecular weight of 527,000 in horses
( Watson et al ., 1991 ). B 48 apolipoprotein is about one-half
the size of B 100 and contains a subset of the B 100 amino
acid sequence (i.e., they are probably coded by the same
gene); however, B 48 is synthesized in the small intestine
and is part of chylomicrons. The origin of the “ 48 ” and
“ 100 ” designations stems from the fact that human B 48
is exactly 48% of the mass of human B 100 . Both B 48 and
B 100 are glycoproteins and have a variety of carbohydrates
attached to them ( Chapman, 1986 ).
In addition to apolipoproteins, HDL contains an additional
protein in the form of the enzyme lecithin:cholesterol
acyltransferase (LCAT), which esterifies cholesterol
esters by transferring an LCFA moiety from phosphatidylcholine
(lecithin) to cholesterol. LCAT is activated by lipoprotein
A-I.
C. Digestion of Fat and Formation of
Chylomicrons
The largest lipoproteins are the chylomicrons, and to understand
their formation, the digestion of triacylglycerol must
be discussed. The main site of digestion and absorption of
triacylglycerol is the small intestine, and the chief enzyme
involved is pancreatic lipase. The pancreas not only supplies
lipase to attack triacylglycerol but also supplies cholesterol
esterase to hydrolyze cholesterol esters and phospholipase
A 2 to attack phospholipids. For any of these enzymes to be
effective, the lipids in food must first be emulsified with
bile. Bile contains micelles composed mostly of bile acids,
phospholipids, and cholesterol. Fats in food become part of
these micelles, and then the enzymes can attack them on
the outer surface of the micelles. The fatty acids, monoacylglycerols,
and cholesterol resulting from the attack of the
enzymes become part of the lipids of the brush border of
the intestinal cells. The intestinal cells then use the monoacylglycerols
and fatty acids to resynthesize triacylglycerol.
Globules of triacylglycerol coated with protein are extruded
from the basolateral membranes into the interstitium as chylomicrons.
The lymphatic capillaries of the microvilli are
called lacteals and have many large openings between the
endothelial cells that line them. Consequently, the chylomicrons
can enter the lymphatics, but not the blood capillaries.
From the small intestine, the lymph flows to larger abdominal
ducts to the thoracic duct and enters the right atrium.
Thus, unlike most other nutrients, most of the absorbed
fat bypasses the portal system and liver ( Gurr et al ., 2002 ;
Vance, 2002b ).
The main apolipoproteins in chylomicrons are A-series,
B 48 , C-series, and E. The A-series and B 48 apolipoproteins
are added by the small intestine, but the C-series and E
apolipoproteins, which are synthesized in the liver, appear
to transfer from HDL to nascent chylomicrons soon after
they are released into the circulation.
Chylomicrons are attacked by lipoprotein lipase, which
resides on the surface of endothelial cells and hydrolyses
triacylglycerol. Most of the resulting LCFA are absorbed
by the tissue cells. As the chylomicron diminishes in size,
some of the apolipoproteins, mostly A-series and C-series,
transfer to HDL. Finally, a much diminished chylomicron
remnant is left and will attach to an apolipoprotein-E receptor
on hepatocytes. The remnant will be absorbed and its
components hydrolyzed within the hepatocytes ( Fielding
and Fielding, 2002 ; Schneider, 2002 ). The transport and
metabolism of chylomicrons are illustrated in Figure 4-6 .
D. Very Low Density Lipoprotein: Synthesis,
Export, and Metabolism
Secretion of very low density lipoproteins (VLDL) into the
plasma is the main method by which hepatocytes export